For a long time, the construction of the medium and low voltage distribution networks is far lagging behind the high voltage power transmission system, the medium and low voltage distribution networks exists problems of weak grid structure and low level of automation. As more and more distributed photovoltaic power access to the power distribution networks, it is bound to produce profound effect on the present relay-protection and automation of the distribution networks. The relay-protection theory of the power distribution networks needs to be improved to promote the performance of the relay-protection.
At present, the power distribution networks has equipped with an active defensive current protection and a passive master station type distribution automatic self-healing control system, which is in order to ensure that the power distribution networks fault isolation and recovery, but it has many problems. The active defensive current protection of the traditional distribution networks usually ensure the selectivity of the action through time coordination. When the protection series is excessive (such as 3 levels), which may lead the time of the protection operation of the last protection series (a circuit breaker of a substation export) is too long (longer than 1 s), the persistent short-circuit current and the resulting voltage collapse may threat the safety of the power distribution networks, the voltage quality and the normal operation of the sensitive power load.
The master station type distribution automatic self-healing control system focus on the coordinated control of the switches, relay-protection devices, etc. Because it is a passive type of the self-healing control after traditional relay-protection action of distribution networks, it not only inherits the characteristic of the traditional overcurrent protection that the action time is too long, but also has the problem of poor accuracy and long-time of “Offline strategy, real-time control”, which eventually lead to a longer time of the self-healing process of the power distribution networks, and not conducive to ensure the quality of power supply of the power distribution networks.
The big data interaction of wide-area layer is one of the development directions of new techniques of the power distribution networks protection. The traditional power communication system cannot meet the demands of the new technical application of the power distribution networks protection for technical system, transmission capacity, bandwidth, real-time, and so on, which restricts the development of new principles of distribution networks protection. SDH technology is the most widely used communication transmission technology in power system, and it is based on the time slot interchange, wherein each service occupies fixed time slot, even if there is no service delivery on the channel, the bandwidth and will not be released, the utilization of the bandwidth is low, and networking flexibility is not enough, data exchange couldn't be achieved with high capacity and high real-time within the scope of the wide area.
At present, some domestic scholars and equipment manufacturers raised solutions of the differential protection and the self-healing of the power distribution networks, which has solved the problems of poor selectivity, long action time, and poor efficiency of the self-healing of the power distribution networks protection in a certain extent.
As an invention patent whose authorized number is CN1167176, is entitled “Relay-Protection of the power distribution networks and Fault Location System”, it disclosed a technique of relay protection of the power distribution networks and fault location. The invention determines whether the device is faulty by detecting and comparing the similarities and differences of the polarities of all the fault component currents flowing through the protected distribution equipment, to determine whether the equipment is fault; and then connecting all relays of the protected device by means of communication, in order to achieve the relay protection of all the distribution lines, power distribution bus and distribution transformers; and then transferring these current polarity information to the dispatch center, so the staff can quickly find the fault location to achieve the fault location rapidly. The patent has the following 3 limitations:
1) The patent is limited to the technical conditions of the power distribution networks at the time, which compares the positive and negative polarity of the current by using the old conventional relay of the beginning of this century, has been unable to adapt to the development of technology in today's distribution networks, and no longer applicable to the construction of the smart distribution networks.
2) The invention patent determines fault sections by comparing the polarity of each polarity comparison relay and determining whether the polarity of each polarity comparison relay is the same (all positive or all negative). The combination of each adjacent elements of relay constitute a large segment protection, but it requires the corporation of a large number of relays to achieve the protection function, incorrect determinations or faults of any polarity comparison relay will lead to protection malfunction or refuse operation, and reduces the four properties of relay-protection.
3) The invention patent need to build a private communications channel between the adjacent elements, in order to compare the polarity of each current, but the its cost too much and its communication rate of is slow.
As an another invention patent whose application number is 201410114922.5, is entitled “Petal-type Distribution Networks Protection and Control System”, it disclosed a kind of petal-type distribution networks protection and control system, the petal-type distribution networks protection and control system includes a master station level, a regional level and a spacer layer; the master station level includes an automated master station, the regional level includes a regional control main station and a centralized protection terminal, wherein the regional control master station is set in the substation, the centralized protection terminal is set in the distribution room, spacer layer includes a centralized protection terminal and a longitudinal protection device, the centralized protection terminal is used to protect all intervals in the distribution room except for incoming lines and outgoing lines, the longitudinal protection device is used to protect incoming lines, outgoing lines and linking-up roads, the regional control master station, the centralized protection terminal and the longitudinal protection device are connected. The patent has the following 3 limitations:
1) In the invention, the centralized protection terminal is equipped with a longitudinal protection device, which requires a dedicated fiber channel constructed between adjacent elements. As element nodes of the power distribution networks is too many, which may lead a substantial increase in the investment of the communications networks, and make it not conducive to the promotion.
2) In the invention, the centralized protection terminal determines fault sections by contrasting the differences in current amplitudes, which is highly required in data synchronization between adjacent elements, the synchronization accuracy error is not allowed to exceed 1 s. Once the synchronization accuracy is not enough, it will lead to a malfunction, meanwhile, it will increase the investment in the synchronization system between adjacent nodes and reduce the economy.
3) In the invention, the centralized protection terminal transport action messages to the regional control main station, and regional control main station conducts self-healing after handing faults, resulting in too many steps long time of the self-healing.
To sum up, the problems of the existing differential protection in the power distribution networks and the self-healing after faults are lagging behind the development of distribution networks technology, the investment of fiber channels for adjacent components is too large, highly dependent on data synchronization, and too many steps of the self-healing. Aiming at the fast fault isolation and self-healing, it is imminent to explore a implementation method of differential protection and self-healing after faults with low cost, principle reliable, and fast action.
As the continuously improvement of communications technology, digital technology, networks control technology, with data processing capability of the power distribution networks terminals and intelligence level, revolutionary changes of the structure and morphology of the power distribution networks protection system have been promoted, which has promoted the organic combination multi-dimensionally and multi-levelly of the relay protection, the self-healing control system and so on, and get the operation information of the wide distribution networks by the functional integration of the same level and multi-level information exchange, to coordinate the coordination between protection and control, and analyzes and evaluates the protection criterion and control strategy, and make the global optimization to improve the existing protection performance, and optimize the control strategy.
Communication technology is an important foundation to achieve the innovation and breakthroughs of distribution networks protection technology. PTN technology supports a variety of two-way point-to-point connection based on packet-switched services, and has the networking capability which is suitable for all kinds of granular business and end to end, which has provided the more suitable “flexible” transmission pipeline for IP service feature and achieve the transport-level business protection and the recovery. Therefore, PTN high-speed data networks technology is more suitable for large amounts of data information exchange, and provides an important guarantee for the realization of wide-area differential protection control system of proactive distribution networks based on phase difference principles.